1. Field of the Invention
The present invention relates to a positioning apparatus for performing feedback control to a motor to stop a control object at a target position.
2. Description of the Related Art
Conventionally, when control of a blow-out port of an automobile air conditioning system is to be performed, i.e., when a control object such as a lever rotated about one point is to be rotated to a target position, an apparatus which gives the target position by a digital value or an analog value to move the control object by a motor to the target position is generally used.
As the conventional positioning apparatus, an apparatus shown in FIG. 1 is known. This positioning apparatus comprises a motor 5 and a PBR (Potentio Balance Resistor) 6 for detecting the present position of the control object to output a present position signal d as a voltage value.
The positioning apparatus further comprises a stop range setting circuit 1 for setting the upper and lower limit values of a stop range having a predetermined width with the target position of a target position signal a input as a digital or an analog value in the center, a comparison circuit 2 for comparing an output from the stop range setting circuit 1 with an output frown the PBR 6, and a drive circuit 4 for performing forward rotation, reverse rotation, and stop of the motor 5 on the basis an output from the comparison circuit 2.
The comparison circuit 2 is constituted by two comparators 21 and 22 using two outputs b and c from the stop range setting circuit I as determination levels, respectively, and a NAND gate 23. When the comparison circuit 2 determines that the control object is out of the stop range, the comparison circuit 2 outputs a out-of-stop range signal e (H(high) output from the NAND gate 23) for permitting the motor 5 to be rotated, and a rotating direction indication signal f for indicating a rotating direction of the motor 5.
The drive circuit 4 rotates the motor 5 in the direction indicated by the rotating direction indication signal f while the out-of-stop range signal e is output from the comparison circuit 2. When the control object reaches a target position being within the stop range by rotation of the motor 5, an output from the PBR 6 has a voltage value set between an upper limit value b and a lower limit value a. For this reason, the out-of-stop range signal a from the comparison circuit 2 is eliminated (output from the NAND gate 23 goes to L(low)), and the drive circuit 4 stops rotation of the motor 5.
With the above operation, the control object is to be stopped within a range having a predetermined width with the target position in the center.
There is a time-lag between determination of drive stop by the comparison circuit 2 after the control object has been set within the stop range and actual stop of the motor 5, and the contro1 object overruns. For this reason, the stop range is set to be larger than an overrunning amount to prevent hunching.
However, in the above conventional positioning apparatus, when the control object is located at a position near the stop range, as shown in FIG. 2, if external noise such as clock noise from a digital circuit is mixed with an output from the PBR 6 indicating the present position, crossing between the line of the upper limit value b or the lower limit value a and the present position signal d is instantaneously repeated. As a result, chattering w is disadvantageously generated by the out-of-stop range signal e from the comparison circuit 2.
When the chattering w is generated by the out-of-stop range signal e, a driver for driving the motor 5 instantaneously repeats an ON/OFF operation within a short time in which the motor 5 cannot be completely rotated. For this reason, the positioning apparatus itself may generate noise. The noise generated by the positioning apparatus does not only adversely affects other peripheral equipment but also enhances chattering of the out-of-stop range signal and amplifies the noise. That is, the positioning apparatus may be placed in a vicious cycle.
As a countermeasure against these phenomena, two conventional countermeasure are generally considered. One countermeasure is a method of setting a hysteresis. That is, after the control object has been set within the stop range once, the width of the stop range is increased to increase the distance between the present position of the control object and the boundary of the stop range.
However, in this case, a range in which the stop position of the control object can be assured is obtained by adding the hysteresis to the stop range, and stop precision is degraded.
The other countermeasure is a method of removing noise. That is, a low-pass filter is inserted into the output or out-of-stop range signal a from the PBR 6.
However, in this case, a time-lag between when the control object is set within the stop range and when the motor 5 is actually stopped increases, and an overrunning amount may increase to generate hunching.